A cellular mobile telecommunication system includes a mobile station communicating with any one of a plurality of geographically spaced base stations. Each base station is located in a pre-defined "cell". The base stations communicate with a mobile switching center which selectively determines which of the base stations and channels should process a call with the mobile station based on considerations such as signal strength between each available channel and the mobile station.
As a mobile station moves from one cell to another, the signal strength diminishes. Therefore, the call must be switched from one base station to another base station. This is referred to as a "handoff". The handoff is ideally implemented without interfering with the call in progress.
When a call is handed off, the handoff may be from one cellular system to another cellular system. Use of a different cellular system can result in different handling of the call. Likewise, capabilities vary among different systems. For example, some systems might include features such as voice privacy, message encryption, automatic retransmission for long messages or the like. It can be helpful for the mobile station to be aware that a change in system or capability has occurred. This could be accomplished by the mobile station automatically requesting what features are supported by making a capability update request. Ideally, this would be done before activating some feature which might not be available. However, sending of a capability request can degrade voice quality at handoff, as there is substantial data transferred in a message.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.